There are two prior constructions which are exemplary of the conventional approach to a dampening system for a printing apparatus, both of which are in use with the standard offset printing machines.
The first of these prior devices incorporates a cloth-covered form roller adapted to contact and feed water to a plate cylinder, a metal oscillator roller mounted to be in continuous contact with the form roller, and to oscillate back and forth in an axial direction in order to promote the even distribution of water on the surface of the form roller, a fountain roller adapted to be partly immersed in water contained in a water fountain, and lastly a cloth-covered ductor roller held at the ends of pivotally mounted swing arms in such a way that the ductor roller can pivot between a first position in which it is in contact with the fountain roller and out of contact with the oscillator roller, and a second position in which it is out of contact with the fountain roller and in contact with the oscillator roller. The ductor roller is typically cloth covered to allow it to absorb a substantial quantity of water from the fountain roller, prior to transfer over to the oscillator roller.
One of the major disadvantages of the arrangement just described is its tendency to prodce an uneven distribution of water on the oscillator, and thus on the form roller to which the oscillator feeds water. The water is transferred in discrete batches, and when the ductor roller is in contact with the fountain roller, no water is being passed to the oscillator roller.
The second conventional arrangement is one in which a form roller is in contact with a plate cylinder, and simultaneously in contact with a transfer roller and with an oscillator roller at two different locations on the form roller. The transfer roller is in turn in contact with a fountain roller which is partly immersed in water contained in a water fountain.
One of the difficulties with this second prior art construction relates to the complexity of the arrangement, and the difficulty of assembly and disassembly.
A particular drawback, which applies to the first of the constructions described above, relates to the limited range of rotational speeds for the fountain roller. The typical prior art construction utilizes a ratcheting system for rotating the fountain roller, thus producing a somewhat intermittent water feed. Further, the ratcheting system is either not adjustable in terms of speed, or only adjustable in a very limited sense, thus not allowing the operator sufficient scope of adjustment to match water feed with water usage during the printing process.